1. Field of Invention
The invention concerns a process for the preparation of sheets, that contains the following characteristics of the process:                extruding a film tube        laying the film tube flat and squeezing it        reversing the film tube        cutting the extruded film tube        
2. Description of the Prior Art
Processes of this kind are used usually in connection with tubular film extrusion plants. Often a film tube is extruded, laid flat, squeezed and subsequently fed to a winding equipment. It is particularly mentionable that the tubular film sheet usually runs through a so-called reversing device after the squeezing.
All afore-mentioned functional units are known in published form. Thus e.g. the DE 100 40 055 lays claim to such a reversing device and also shows the associated squeezing and lay-flat (flattening) devices.
In order to transform the film tube into film sheets one uses different types of cut-off devices that are attached to the winding equipment and transform the tubes into film sheets immediately before the beginning of the winding process. Usually for this purpose trimming sections are arranged at both the edges of the collapsed film tube. This process however results in a lot of wastage. Since modern standard procedures however allow for an exact adjustment of the diameter of the extruded tubular film and the breadth of the collapsed film tube, in many cases a trimming section is not used. Instead of that cleavers are attached to the winding equipments that slit open the collapsed tubular sheet directly at its edges and thus avoid unnecessary wastage.
This type of the production of sheets from film tubes is however not suited for thin sensitive or sticky films. These films are affected by the action of the cleaver on the collapsed film tube. Films of the aforementioned type are thus still prepared with the help of trimming sections that are put on the film after the film has run through the reversing device.